Michael Schumacher's 14-year-old son Mick, who was skiing with his father at the time, was also being questioned by investigators.

Grenoble, France: A camera fixed to the ski helmet of Michael Schumacher is being inspected by investigators probing the accident that has left the German retired racing legend comatose in a French hospital with critical head injuries.

A source close to French authorities handling the investigation disclosed the existence of the camera late Friday.

The source said it had been taken to see if it can yield any clues as to the circumstances of the accident.

Schumacher's 14-year-old son Mick, who was skiing with his father at the time, was also being questioned by investigators, the source said, confirming information reported by the French newspaper Dauphine Libere.

The developments in the probe came the day Schumacher turned 45 while still in an induced coma in a hospital in the French Alpine city of Grenoble after his December 29 accident.

Fans marked the birthday with a silent vigil outside the facility, part of which was organised by Ferrari, Schumacher's former team.

French authorities are examining various factors in the ski accident, in which Schumacher's head hit a rock so hard his helmet was split in two.

Although he was conscious when airlifted from the unmarked run in the Meribel resort, where he owns a property, Schumacher was agitated and soon fell into a coma, prompting his transfer to Grenoble hospital.

He has undergone two operations to remove blood and pressure from his brain. His manager said in the last update on Wednesday that he remains in a critical but stable condition.

His family is at his bedside, including his wife Corinna, his two teenage children, his father Rolf and brother Ralph, who is also a racing driver.

Fans mark sad birthday

The accident has shocked legions of fans used to seeing the seven-time Formula One world champion cheat death on the race track.

Ferrari, which brought in the fans from Italy and elsewhere in France on board two buses, put a message on its website, saying "Forza Michael," or "Go Michael" in Italian.

"He is tackling the most important fight of his life and therefore we want to send him very special wishes," said Schumacher's old team, as similar messages poured in from around the world.

German former tennis ace Boris Becker wished his compatriot "Happy Birthday" on Twitter, adding "Fight Schumi" in a German hashtag.

The family responded in a statement Friday by saying "the incredible sympathies shown today by the Ferrari fans outside the hospital has utterly overwhelmed us and moved us all to tears".

"We are deeply grateful for it and also for all the heartwarming and heartfelt wishes for Michael to get well soon, which have reached us from all over the world."

Stefano Pini, a 47-year-old accountant from Milan who arrived to pay tribute, said: "Schumi has given us a lot in the past. The least we could do is to come here and support him and his family on his birthday."

However, the Ferrari tribute created controversy, with some accusing the Italian racing giant -- which asked fans to sport its trademark colour red and its insignia -- of bad taste. (Related: Hamilton slammed for skiing pictures)

Accident probed

Questions have emerged over exactly how the accident happened on a small, seemingly innocuous off-piste section of Meribel located between two ski slopes -- one classed as easy and the other as intermediate.

The camera could be valuable in providing information. Schumacher's manager, Sabine Kehm, refused to comment on what it might contain or where it has been kept since the accident.

"That is among several things about which I will not comment," she said.

Schumacher's son, and a friend of his who was also skiing at the time, were being asked to give eyewitness testimony.

Prosecutors are also looking at whether the limits of the pistes next to the accident site were correctly marked, and whether the safety releases on Schumacher's skis operated properly.

There have been conflicting statements about the speed Schumacher was going when he crashed.

Schumacher, who made his debut in 1991, dominated Formula One during his career, winning more world titles and races than any other driver.

He retired definitively in 2012, at the end of a disappointing two-year comeback from an earlier attempt to quit racing.